Capital gains tax

Tax Planning and Financial

Hi,
I have owned the property we rent out for over 20 years and lived in it for all apart from the last three years as my main home.
I’ve since moved into my new wife’s house a few years ago and we rent out my former home to tenants.

I’ve been told by an accountant friend that I’m liable to pay capital gains on the whole property uplift from when I first purchased 20 years ago rather than just the period I’ve been renting it out. He calculated this at almost 40k which is more than we have earned in renting it for the past few years.
Has anybody any knowledge of this or can somebody recommend a company that I can approach to find out the real liability?